The present invention relates to a method for producing high melting point crystals (hereinafter referred to as xcex2-crystals) of ethyl(R)-2-[4-(6-chloroquinoxalin-2-yloxy)phenoxy]propionate (hereinafter referred to as quizalofop-p-ethyl) and xcex2-crystals thereby produced.
Quizalofop-p-ethyl which is useful as an active ingredient for a herbicide, is known to have two types of crystal forms, i.e. low melting point crystals (hereinafter referred to as xcex1-crystals) and xcex2-crystals. JP-B-4-76721 discloses that xcex2-crystals can be produced by gradually cooling a solution having quizalofop-p-ethyl dissolved in a solvent, with stirring and maintaining it at a crystallizing temperature, and the produced xcex2-crystals are ones having crystallites of at most 1 xcexcm agglomerated, whereby in some cases, transfer of the solvent slurry, or filtration and drying of the precipitated crystals, are difficult.
It is an object of the present invention to provide xcex2-crystals which are easy to handle in an industrial scale and a method for their production.
The present invention provides a method for producing xcex2-crystals, which is characterized by heating quizalofop-p-ethyl in the form of xcex1-crystals, or xcex1-crystals and xcex2-crystals, within a range of from 50xc2x0 C. to lower than the melting point of xcex1-crystals, and xcex2-crystals produced by such a method.
The heating can be carried out by heating in contact with a high temperature innert gas (hereinafter referred to as a gas-solid heating method), heating with mixing in a solid state (hereinafter referred to as a solid heating method) or heating in suspension with a hardly soluble solvent (hereinafter referred to as a suspension heating method).
The gas-solid heating method may, for example, be a method wherein quizalofop-p-ethyl is charged into an apparatus equipped with a gas-solid contact function such as a pneumatic conveying dryer, a fluidized bed dryer, a circulation dryer or a hot air dryer, and a high temperature gas such as heated air or heated nitrogen is blown thereinto for heat treatment.
The solid heating method may, for example, be a method wherein quizalofop-p-ethyl is charged into an apparatus equipped with a heating means such as a jacketed kneader, an agitation drier, a jacketed screw conveyer or a conical drier and provided with a mixing function, and heated and mixed for a predetermined period of time.
The suspension heating method may, for example, be a method wherein quizalofop-p-ethyl is charged to a hardly soluble solvent such as water or ethylene glycol, and heated in suspension.
The heating can be carried out usually at a temperature of from 50xc2x0 C. to lower than the melting point of xcex1-crystals, preferably from 65xc2x0 C. to lower than the melting point of xcex1-crystals.
The heating time usually depends on the heating temperature and is required to be a time until xcex1-crystals have substantially disappeared. For example, it takes at least 100 hours when heating is carried out at 60xc2x0 C. and a few hours when heating is carried out at 70xc2x0 C., to obtain xcex2-crystals.
After xcex1-crystals have been converted to xcex2-crystals by heating, cooling, or cooling followed by filtration in the suspension heating method, is carried out to obtain xcex2-crystals.
In the gas-solid heating method and the solid heating method, xcex2-crystals can be obtained in a solid state, and can be used as it is, or after pulverization, for example, for the production of an aqueous suspension concentrate.
Further, in the gas-solid heating method or the solid heating method, if quizalofop-p-ethyl mainly in the form of xcex1-crystals produced by crystallization method, is employed, the resulting xcex2-crystals will be obtained in a powder form without forming a block solid in the above apparatus, whereby high productivity can be maintained. In such a case, it is preferred to employ quizalofop-p-ethyl mainly in the form of xcex1-crystals containing substantially no solvent for crystallization. JP-B-2-214504 discloses that xcex1-crystals obtained as a wet product by a crystallization method may sometimes change into xcex2-crystals when dried at a temperature exceeding 70xc2x0 C. for at least 5 hours. However, if such a wet product containing a solvent for crystallization is used, dissolution by the solvent takes place during the heating, and the product is likely to be an aggregated solid in the apparatus, whereby industrial operation is practically difficult.
In the suspension heating method, removal of the hardly soluble solvent may sometimes be required. For example, when the hardly soluble solvent is water, a drying step may sometimes be required. However, in a case where an aqueous suspension agricultural chemical composition is to be obtained, drying will not be required depending upon the weight concentration of the suspension. Further, by the suspension heating method, the dispersed state in the apparatus can be excellently maintained during the heating operation, whereby the temperature control of quizalofop-p-ethyl is easy, and uniform xcex2-crystals can be produced efficiently. Further, xcex2-crystals formed by the suspension heating method are excellent in the filtration property and easy for drying.
Now, a method for obtaining desired xcex2-crystals by suspending the starting material in water which is the hardly soluble solvent and which is industrially inexpensive, will be described.
Namely, quizalofop-p-ethyl in the form of xcex1-crystals, or xcex1-crystals and xcex2-crystals, is suspended in water usually in an amount of from 0.1 to 60 wt %, preferably from 1 to 60 wt %, and the suspension is maintained at a temperature of from 50xc2x0 C. to lower than the melting point of xcex1-crystals until xcex1-crystals substantially disappears, preferably at a temperature of from 65xc2x0 C. to lower than the melting point of xcex1-crystals for from 10 minutes to 48 hours. Thereafter, it is cooled to a temperature where usually filtration can be carried out, usually at most 50xc2x0 C., preferably at most 40xc2x0 C., followed by filtration to collect crystals. When the crystals have been completely converted to xcex2-crystals, drying of the crystals can be carried out at a temperature lower than the melting point of xcex2-crystals, usually lower than the melting point of xcex1-crystals, preferably at most 65xc2x0 C.
Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. The weight ratio of xcex1-crystals and xcex2-crystals approximates the area ratio of the respective endothermic peaks obtained by the differential scanning calorimetry and thus was obtained from the area ratio of the respective endothermic peaks.